18: officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent as well as the island of Tasmania and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area. Neighboring countries include Indonesia, East Timor and Papua New Guinea to the north; the Solomon Islands, Vanuatu and New Caledonia to the north-east; and New Zealand to the south-east. | AUSTRALIA | Happy 30th Birthday Zach! Nov 11, 2012.